Without writing a book. From what I have read, they ran casing and cemented it in a well bore that they had lost circulation. At that point people relax and think it's done nothing else to worrie about but if the mud in the hole was not circulated prior to the cement being pumped, hydrocarbon gasses still in solution could have been circulated to a point close to surface (inside the riser above the BOP stack) and come out of solution (bubble point) and become a free gas. At this time, the released gasses will expand exponetially emptying the riser of mud and allowing the gas to reach the rig floor. At the same time, the reduced hydrostatic pressure will allow more formation fluids to enter the well bore adding fuel to the fire when it reaches surface. If the rig crew did not respond by shutting in the well quickly enough, the gasses could have ignited on the rig floor causing explosions preventing any further attempts at shutting the well in.